In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date:    (i) part of common general knowledge; or    (ii) known to be relevant to an attempt to solve any problem with which this specification is concerned.
The majority of scientific opinion is to the effect that global warming (also known somewhat euphemistically as “climate change”) is real, and could have devastating environmental and economic consequences. Global warming is caused by ever increasing levels of carbon dioxide present in the atmosphere, which are released when fossil fuels are burned, such as to produce electricity. When burned, carbon which has been stored in the fossil fuel (such as coal) for many thousand of years is converted to carbon dioxide and released into the atmosphere.
Ways to make coal burn “cleaner”, in terms of the amount of carbon dioxide released into the atmosphere, are being investigated, as well as ways of “sequestering” carbon dioxide, such as by storage in underground or undersea geological formations.
Power generators that are fired by the combustion of biogases, such as methane, may contribute less to global warming than coal-fired powered stations. As discussed in published German patent specifications DE4440750 (Schirmer Umwelttechnik GMBH) and DE 19715646 (Paesler), and published Japanese specifications JP2002361217 (Kubota KK), biogases that are suitable for use in this context may be produced as a by-product from processing of animal and/or municipal waste through processes such as anaerobic fermentation.
Power-generation facilities that release negligible carbon dioxide have also been investigated. For example, solar and wind power-generation facilities are used in many parts of the world. Although eminently cleaner than power-generation systems based on burning fossil fuels and biofuels, they are far less efficient, and depend on the existence of particular environmental conditions (i.e. sunlight and wind). Of course the times when such conditions are not present are effectively “down time” for the power-generation system.
Power-generation facilities that do not increase the net amount of carbon dioxide in the atmosphere have also been proposed. Power-generation facilities based on biomass are an example and are already in use in some parts of the world. Biomass is a naturally produced organic fuel that may be specifically grown as fuel (such as wood chips) or obtained as a waste product from other processes, such as “green waste” from agricultural processes.
Biomass is burned in the same way as fossil fuel to produce heat to power a boiler, which in turn drives steam-powered turbines to generate electricity. Other uses of biomass to produce energy are also known such as for making bio-diesel, and its use in gasification processes.
Although burning biomass releases carbon dioxide into the atmosphere, the net amount of carbon dioxide does not increase, because of the removal of carbon dioxide from the atmosphere due to the occurrence of photosynthesis as the biomass was grown.
Biomass power stations are promising in that they do not rely on particular environmental conditions as do solar and wind energy. Biomass is also a renewable energy source which can be easily grown and/or obtained from waste products.
An example of a biomass power system is given in published United States Patent Application No US2004/0129188, which describes a power-generation system where biomass—which is grown in a biomass field near a furnace—is periodically harvested and burned in the furnace to power a generator. The goal of the system is to obtain a “partially closed loop”, to which only sunlight and air are added and in which a biomass field supplies all the fuel needed to produce the desired amount of electricity. The goal is said to be achieved by recycling waste heat and other by-products of biomass combustion, such as carbon dioxide, water vapour and ash, back into the biomass field as input energy and nutrients.
Whilst the system has advantages over traditional biomass power stations there nevertheless remains a need for a system that makes more effective use of the by-products of biomass combustion than is described in the above document.
WO 2005/005786 describes a process where excess airflow is introduced during biomass combustion to increase nitrogen oxide and nitrogen dioxide production (referred to as NOx). In addition, the oxidation area of the boiler is operated at a temperature exceeding 2000° F. and flue gases are maintained in the high temperature environment for an increased residence time to further increase NO and NO2 production. Flyash from the incinerated biomass is then filtered from the flue gases exhausted from the boiler, with NOx removed from the filtered flue gas and used in the production of a nitrogen fertiliser. The nitrogen fertiliser itself is also blended with the flyash to form a “balanced fertiliser product” used to grow additional biomass material.
Again, whilst the system described in the above document has advantages over traditional biomass power stations, there nevertheless remains a need for a system that makes more effective use of the by-products of biomass combustion than is described in that document.